These results suggest that the plasma levels of IL-6 reflect the severity of MDD and that plasma IL-6 levels might be another biological-state marker for the depressive state.
Here we examined circulating concentrations of inflammatory cytokines (IFN-γ, TNF-α, IL-1β, IL-6), and the acute phase protein CRP alongside plasma tryptophan, kynurenine, kynurenic acid (KYNA) and 3-hydroxyanthranilic acid (3-HAA) concentrations, and whole blood mRNA expression of IDO, kynurenine aminotransferases (KAT I and II), kynurenine-3-monooxygenase (KMO), kynureninase and SERT in patients with major depressive disorder (MDD) compared with age and sex-matched controls.